Processes for producing methanol from hydrogen and carbon oxides are based on the following two reactions: EQU CO+2H.sub.2 =CH.sub.3 OH EQU CO.sub.2 +3H.sub.2 =CH.sub.3 OH+H.sub.2 O
Accordingly, in the synthesis of methanol, if the relation (R) among the mol percentages of CO, CO.sub.2 and H.sub.2 is 2.0 as calculated according to the following equation, the relation is regarded to be stoichiometric: EQU R=(H.sub.2 mol %-CO.sub.2 mol %)/(CO mol %+CO.sub.2 mol %)
Consequently, it is common to think that a synthesis gas, which has a composition very close or equivalent to the aforesaid stoichiometric composition demanded for methanol synthesis, namely, a composition that makes R=2.0, should be produced.
As a conventional process, Japanese Patent Publication No. 46961/1980 discloses a process which comprises dividing a feedstock gas into two streams, subjecting one of the streams to conventional steam reforming to form an effluent gas, combining the effluent gas with the other stream, and subjecting the combined stream to secondary reforming using oxygen, thereby producing a synthesis gas having the stoichiometric composition (R=2.0) suitable for methanol synthesis. Further, the Japanese Patent Laid-Open Publication No. 3614/1990 proposes to use the enthalpy of a product from the secondary reforming using oxygen, as a heat source for the primary reforming by steam.
However, when R is made 2.0 as in the above prior art processes, loss of energy is increased due to increase in the consumption of oxygen, though the energy for compressing the synthesis gas is reduced.
Further, as a process to solve the problems of the above prior art processes, Japanese Patent Laid-Open Publication No. 4140/1985 teaches a process for producing a methanol synthesis gas from natural gas as a feedstock through primary steam reforming and secondary oxygen combustion reforming, wherein the primary reforming conditions, secondary reforming conditions and amount of oxygen supply are so created that the synthesis gas may have a composition of R=2.2-2.5 and in consequence the energy efficiency of the whole plant including oxygen production, synthesis gas production and methanol synthesis becomes higher than that of the case of R=2.0.
However, this process involves a problem of limits in fabricating the combustion-type steam reforming furnace used in the primary steam reforming and a problem of reliability for the uniform distribution of the feedstock into a large number, such as more than a thousand, of reaction tubes.
Further, the aforesaid process of Japanese Patent Laid-Open Publication No. 3614/1990 has no combustion system for effectively burning purge gas from the synthesis loop as a fuel within the process system. Therefore, it needs new installation of a gas turbine and requires large plant investments to maintain the efficiency of the system. In addition, the process cannot maintain its efficiency unless it sells excess heat to the outside by way of means such as exportation of power as occasion demands.
Plants for producing methanol from natural gas have recently been demanded to reduce costs with economical efficiency added and to further improve the energy efficiency of the whole methanol synthesis plant, in the construction of methanol plants that are significantly increasing in size due to the production of fuel methanol.